1. Field of the Invention
The present invention relates to a voltage generator and bandgap reference circuit, and more particularly, to a voltage generator and bandgap reference circuit with reduced layout area and high-accuracy reference voltage.
2. Description of the Prior Art
A stable reference voltage source or current source immune to temperature variation, e.g. a bandgap reference circuit, is usually applied in an analog circuit to provide a reference voltage or current, for maintaining accurate operations of a power source or other circuits. In short, a bandgap reference current source mixes currents/voltages of a proportional to absolute temperature (PTAT) and a complementary to absolute temperature (CTAT) with a proper ratio, to cancel out the PTAT and CTAT components, and generates a zero temperature correlated (zero-TC) current/voltage.
In detail, please refer to FIG. 1, which is a schematic diagram of a bandgap reference circuit 10 according to the prior art. The bandgap reference circuit 10 includes an operational amplifier (OP) 100, bipolar junction transistors (BJTs) Q1, Q2, and resistors R1, R2, R3. As shown in FIG. 1, since input voltages VX and VY of positive and negative input terminals of the OP 100 are identical in the bandgap reference circuit 10, i.e. VX=VY=VEB1 and VEB1 is a base-to-emitter voltage of BJT Q1, a PTAT current Iptat can be generated via the resistor R3 and a voltage difference between voltages VY and VZ as shown in Eq. 1, where VZ=VEB2 and VEB2 is a base-to-emitter voltage of BJT Q2, and R3 represents the resistance of the resistor R3.
                    Iptat        =                                            VY              -              VZ                                      R              ⁢                                                          ⁢              3                                =                                                                      VEB                  ⁢                                                                          ⁢                  1                                -                                  VEB                  ⁢                                                                          ⁢                  2                                                            R                ⁢                                                                  ⁢                3                                      =                                                            V                  T                                ⁢                InK                                            R                ⁢                                                                  ⁢                3                                                                        (        1        )            where K denotes that the BJT Q2 can be taken as K pieces of BJT Q1 connected in parallel. Referring to Eq. 1, since a thermal voltage VT is PTAT, the PTAT current Iptat carried by the resistor R3 is also PTAT.
Since the base-to-emitter voltage VEB2 of the BJT Q2 contains a CTAT, Vout denotes the output voltage of the bandgap reference circuit 10 as shown in Eq. 2:
                              Vout          =                                                                                          V                    T                                    ⁢                  ln                  ⁢                                                                          ⁢                  K                                                  R                  ⁢                                                                          ⁢                  3                                            *                              (                                                      R                    ⁢                                                                                  ⁢                    2                                    +                                      R                    ⁢                                                                                  ⁢                    3                                                  )                                      +                          VEB              ⁢                                                          ⁢              2                                      ⁢                                  ⁢                                            ∂              Vout                                      ∂              T                                =                                                                                          R                    ⁢                                                                                  ⁢                    2                                    +                                      R                    ⁢                                                                                  ⁢                    3                                                                    R                  ⁢                                                                          ⁢                  3                                            *                                                ∂                                      V                    T                                                                    ∂                  T                                            *              ln              ⁢                                                          ⁢              K                        +                                                            ∂                  VEB                                ⁢                                                                  ⁢                2                                            ∂                T                                                                        (        2        )            
Referring to Eq. 2, the term
      ∂    Vout        ∂    T  can be set equal to zero by choosing a suitable value of K and suitable resistances of the resistors R2 and R3, such that the bandgap reference voltage Vout is a zero-TC voltage.
However, the conventional bandgap reference circuit using BJTs for performing temperature compensation usually requires a higher power supply voltage and a higher reference voltage, which leads to large static power loss and is improper for applications requiring lower voltage. Meanwhile, the conventional bandgap reference circuit using BJTs also needs a large layout area. Consequently, manufacturers have provided a bandgap reference circuit using complementary metal oxide semiconductor (CMOS) for temperature compensation; however, a CTAT voltage generated by such circuits varies as manufacturing processes vary, and accuracy of the zero-TC voltage is also reduced. In such a situation, the prior art has to be improved.